1. Field of the Invention
The present invention is directed to a liquid quenching method for manufacturing an amorphous metal wire or thin strip (hereinafter "thin strip") by quenching and solidifying a molten alloy on a moving cooling substrate. More particularly, the present invention relates to a method for supplying molten alloy from a ladle storing the molten alloy to a tundish.
2. Description of the Prior Art
Liquid quenching methods for producing thin strips include, for example, the single roll method which discharges a molten alloy on to a single cooling roll rotating at a high speed resulting in the manufacture of a thin strip. In the twin roll method, the molten alloy is discharged between a pair of cooling rolls rotating at a high speed resulting in the manufacture of a thin strip.
A liquid quenching method which uses a single roll cooling/solidification apparatus, as shown in FIG. 7, will be explained. Molten alloy 6 is poured into a tundish 5 so that the level of the molten metal becomes constant. Twyer bricks 9 are disposed on the bottom wall of this tundish 5. An intermediate nozzle 10 and a nozzle holder 11 are interconnected to a passage 13 bored in these twyer bricks 9 to provide a fluid path for the molten alloy. An expanded internal space 14 is located inside the nozzle holder 11. A nozzle chip 12 is fitted to the distal end of the nozzle holder 11, and a nozzle slit 15 is inserted inside this nozzle chip 12 for discharging molten alloy onto the chill roll 8. The expanded internal space 14 inside the nozzle holder 11, the nozzle chip 12 and the nozzle slit 15 are illustrated in FIG. 8. Here, the expanded internal space 14 represents an expanded portion of the molten metal passage 13 inside the nozzle holder 11 so as to obtain a thin strip having a large width. The nozzle slit 15 provides an opening for jetting the molten metal flowing through the nozzle chip 12.
When a tundish stopper 4 is moved up, the molten alloy 6 inside the tundish 5 is allowed to flow through the molten metal passage 13 and is jetted from the nozzle slit 15 onto the cooling roll 8. At this time, the flow rate of the molten alloy 6 flowing out from the nozzle slit 15 onto the cooling roll 8 is controlled in accordance with the static pressure of the molten metal inside the tundish 5. The molten alloy 6 jetting out from the nozzle slit 15 is rapidly cooled on the surface of the cooling roll 8 and is formed into the thin strip 7.
The cooling roll 8 is illustrated in a small scale compared with the large scale of the tundish 5 in FIG. 7 in order to make the entire apparatus more easily understood.
In order to obtain the thin strip by either of the liquid quenching methods described above, the cooling rate must be set to at least about 10.sup.2 K/sec. Therefore, there is a limitation on the sheet thickness of the resulting thin strip. It is as small as less than about 0.1 mm. When the thin strips having a thickness of less than 0.1 mm are produced by the liquid quenching method, there are differences in the limiting conditions of the various production factors in comparison with ordinary ingot casting methods and continuous casting methods according to conventional solidification technologies. The most important limiting condition is the feed quantity of the molten alloy. In the case of the continuous casting methods for steels, etc, that have been ordinarily employed, the quantity of the molten alloy that can be provided to a casting mold is several tons per minute. A greater quantity of molten alloy can be provided in ordinary ingot casting methods.
In contrast, in the liquid quenching method which is the subject of the present invention, the feed quantity of the molten alloy must be reduced to a very small quantity of not greater than 100 kg/min. This is because of the limitation on the thickness of the thin strip. The maximum strip thickness that can be ordinarily obtained by the single roll method, for example, is about 0.1 mm. The peripheral speed of the cooling roll in this case is about 10 m/sec and the maximum width of the thin strip is about 200 mm. In the case of alloys containing iron as the principal component, the feed quantity of the molten alloy must be controlled to about 90 kg/min.
When the thin strip is produced by the liquid quenching method in an industrial practice, it is a very important to minimize the feed quantity of the molten alloy.
In the case of a conventional continuous casting method, for example, the molten alloy is supplied from a ladle to the casting mold through a tundish. In this instance, a system using a ladle stopper fitted to a long nozzle hole at the bottom of the ladle is employed as one of the methods of controlling the feed quantity of the molten alloy. In other words, the feed quantity of the molten alloy is controlled by moving the ladle stopper up and down, thereby controlling an opening area of the long nozzle. Since a conventional continuous casting method can supply a large quantity of the molten alloy such as several tons per minute as described above, the feed quantity can be easily controlled by such a stopper system.
In contrast, in the case of the liquid quenching method, which is the object of the present invention, the feed quantity of the molten metal must be limited to not greater than 100 kg/min. Therefore, it becomes difficult to employ, as such, the stopper system described above. Japanese Unexamined Patent Publication (Kokai) No. 1-34550, for example, proposes a method which uses the stopper system in the liquid quenching method. Though this method is not limited to the production of the amorphous alloy thin strip, it is devised so as to reduce the relative feed quantity of the molten alloy. It measures the weight of the molten alloy inside the tundish during charging and controls the up or down moving speed of the ladle stopper and the ladle stopper position on the basis of this measurement so as to control the feed quantity of the molten alloy. This method limits the lower limit of the moving distance of the ladle stopper to 2 mm and the upper limit to 6 mm. It can control the feed quantity of the molten alloy with a very high level of accuracy.
According to this method, however, the weight of the tundish must be measured during charging and hence, the control becomes complicated. Further, because a measuring instrument and a computer must be installed, the setup cost becomes high and thus the production cost becomes high. If the moving distance of the ladle stopper is limited to an excessively small value, the operation becomes more difficult because most installations are not free from vibrations no matter how precise they may be. Because of vibration problems, the moving distance of the ladle stopper must be at least about 5 mm.